Susan WeigelOne of gardening's biggest mysteries is why some plants flower and others don't in a given year.

Q: I'm wondering why one of our three American dogwoods failed to bloom this year. The others bloomed just fine. Four of our five cherry laurels failed to bloom. The other was fine. Any ideas what might be going on?

A: That kind of stuff drives gardeners (and horticulturists) crazy. You'd think that if something were wrong, they'd all not bloom.

Figuring out the "why" in cases like that call for some Plant CSI (crime scene investigation) and a bit of botanical deduction (i.e. guessing).

The first thing I do is look around the crime scene for clues. Sometimes the non-bloomers will be just close enough to nearby big trees that they lack the energy to flower because the trees are throwing off shade or their big roots are stealing nutrients from the soil.

I had a case once where a Camp Hill fellow was losing Japanese hollies, and all of the soil tests and disease checks were coming back negative. When I had a look, I noticed a big stump. Turns out he had a big oak removed, and that suddenly let full sun into the area where the hollies were growing. The new heat and sun shocked the hollies, which had adapted to the shade for some 20 years. The plants were basically frying in a pattern starting with the most sunny to the least. Ones that were still in the shade (from other trees) were unfazed.

I'd also look for clues related to stresses on those particular struggling plants. Any signs of bug damage? Signs of disease? A biggie in my mind is drainage. We had a lot of soggy spots for the first time last year, so areas that ended up soggy for a few weeks last fall could've led to some root-rotting. In bad enough situations, that can kill plants -- including the following season. But in marginal diebacks, it can cause enough stress that the plant foregoes reproduction (which flowering is a function of) and focus on surviving by redirecting energy to root regrowth. Severe heat or drought can cause a similar reaction at the opposite end of the scale.

If none of that checks out, I'd then consider the oddball weather we've had this year. Most plants were running about 3 weeks ahead of schedule this winter and spring. We then had a few frosty nights that burned off the tender young foliage of some plants. It's also possible that the flower buds were harmed, which would abort the flowers. Again, I'd look for clues as to why that would only affect some of your plants. If the cherry laurel that bloomed was closest to the house or, say, a brick or rock wall that's enclosed to afternoon sun, that microclimate might have been enough to salvage flower buds while more cold-exposed ones would've croaked.

If none of that explains it, the last thing I'd think about would be soil nutrition. Sometimes soil ends up imbalanced, often with an excess of nitrogen (aids foliage growth at the expense of flowers) because people are fertilizing nearby lawns several times a year with high-nitrogen lawn fertilizer. Other times soil just lacks important nutrients that plants need to flower well, especially phosphorus and potassium.

A soil test will nail this one down. Do-it-yourself kits are available at county Extension offices and most garden centers ($9-$10) or online at http://www.aasl.psu.edu/SSFT.HTM. The results will tell you if you're lacking something and what to do about it.

A lot of times, non-bloom fixes itself. If the stresses go away or the plant strengthens itself, it'll resume blooming even when you don't do anything. For chronic non-bloomers that don't respond to anything I do, they get "shovel-pruned" out of my yard. In other words, put out or get out...